1. Field of Invention
The present invention relates to an active matrix liquid crystal display device using a thin film transistor and its manufacturing method.
2. Description of Related Art
From the past, there have been known active matrix liquid crystal display devices and liquid crystal panels of a structure having interposed twisted nematic (TN) liquid crystal or ferroelectric liquid crystal, used as an optically modulated material, between a thin film transistor substrate and an opposing substrate. In these liquid crystal display devices, a thin film transistor (TFT) and a pixel electrode selectively driven by the TFT are provided on a thin film transistor substrate, and the opposing electrode is provided on the opposing substrate.
FIG. 1 shows one example of a conventional liquid crystal display device. The size of the thin film transistor in the drawings is shown larger than its actual size in order to clearly represent the structure of the thin film transistor. Liquid crystal 103 is interposed between thin film transistor substrate 101 and opposing substrate 102. Opposing substrate 102 are formed a black matrix 104 composed of a light-blocking film such as chrome, and red, green, and blue color filter sections 105, 106, 107 formed by gelatin dyed with red, green, and blue color. On these are formed a protective insulation film 108 and an opposing electrode 109 composed of a transparent conductive film. Meanwhile, on the inside of thin film transistor substrate 101 are formed a thin film transistor 115 constituted by a gate insulation film 110, gate line 111, source line 112, interlevel insulator film 113, and contact hole 114, and a selectively driven pixel electrode 116 composed of a transparent conductive film. Alignment films 117 and 118 are formed on pixel electrode 116 and opposing electrode 109, and they are applied with rubbing processing. On source line 112 is formed an insulation film 119 composed of a protective film, and this insulation film is removed on the top of pixel electrode 116, opening a window. Thus, source line 112 can be protected. In addition, the insulation film helps prevent the reduction of voltage applied to the liquid crystal. The distance between pixel electrode and opposing electrode is generally called a cell gap, and it is a parameter that greatly controls the optical properties. This cell gap deviates more easily as the liquid crystal panel becomes larger. Therefore, uniformity is maintained in a large liquid crystal panel by using a gap member (spacer) 120.
However, there are four major problems in the above-mentioned previous technology, as described below.
First, when matching a thin film transistor and an opposing substrate, it is necessary to take a large margin for alignment of the pixel electrodes formed on the thin film transistor and the color filters and black matrix formed on the opposing substrate. This greatly reduces the aperture size. It is understood that this problem can be improved somewhat by forming the black matrix on the thin film transistor substrate, and various companies are conducting examinations. For example, in Japanese Laid-Open Patent No. 2-207222, a method is disclosed whereby a light-blocking film for becoming a black matrix is formed on a thin film transistor substrate.
When forming a black matrix on the side of the thin film transistor substrate, a resist having a light-blocking material added, such as black resist is considered to be the most superior material to compose the black matrix. However, use of a resist presents three further problems.
Problem 1-1 is possible contamination from Na, and the like. There are known black resists having mixed red, green, and blue dyes, but these resists contain a large quantity of impurities. Thus, there is a concern of degradation of thin film transistor properties due to Na contamination.
Problem 1-2 is alignment defects. The relationship between film thickness and the light-shielding property in a black resist such as that mentioned above is shown in FIG. 2. In order to obtain a more sufficient light-blocking property (light transmissivity less than or equal to 1.5%, OD value greater than or equal to 1.8), it is clear that at least a film thickness 1.5 .mu.m or more becomes necessary. In this case, rubbing can no longer be performed well due to the difference of levels, and alignment defects of the liquid crystal are caused following the pattern of the black matrix.
On the other hand, specific resistance is also important. When trying to form a pattern so as to cross the space between pixel electrodes, it is required that the specific resistance be sufficiently high in relation to the liquid crystal. Also, the light-blocking property and specific resistance are reciprocal, and establishing both of them is difficult. The specific resistance in the above-mentioned black resist depends also on the film thickness, but it is about 10.sup.8.OMEGA. cm, and it does not meet the specification.
Alignment defects may also be caused by the processed shape of the black resist. An example of that is shown in FIG. 3. The black resist has an overhang structure, and the part under the overhang (point A of FIG. 3) is in a location where rubbing is difficult to perform.
Problem 1-3 is the increase of number of processes. Even the number of masks necessarily on the thin film transistor substrate increases by one sheet.
The second major problem of the previous technology is the degradation of properties of a thin film transistor due to ultraviolet light. FIG. 4 is a comparison of the properties of a thin film transistor before and after ultraviolet irradiation. It is clear that the properties are degraded due to irradiation by ultraviolet radiation as shown in FIG. 4. Ultraviolet-hardened adhesives are used widely when matching a thin film transistor substrate and an opposing substrate. Also, because the thin film transistors are also exposed to ultraviolet radiation during actual use, ultraviolet radiation has become a great obstacle against assuring reliability of thin film transistors.
The third major problem is the use of spacers. If the process of scattering spacers can be omitted, an increase in throughput and a reduction of cost can be realized.
The fourth major problem is the etching of anti-static wiring. Although it was not explained in the above-mentioned previous technology, in order to protect yield from static electricity when forming a thin film transistor substrate, a method is used whereby the gate line patterns are first shorted, and then separated. Nevertheless, this creates an undesired increase in the number of processes.